1. Field of the Invention
The present invention relates in general to seats for motor vehicles and more particularly to seats of a shape-adjustable type which can change the shape of the seat to fit the occupant. More specifically, the present invention is concerned with shape-adjustable seats of a type in which the shape change of the seat is achieved by controlling air pressure in various air-bags installed in the seat.
2. Description of the Prior Art
Hitherto, various types of shape-adjustable seats have been proposed and put into practical use particularly in the field of motor vehicles.
One of them is disclosed in Japanese Patent First Provisional Publication No. 61-257333. The seat of this publication employs a plurality of air-bags which are installed in given portions of the seat. By changing the air-pressure in each air-bag, the shape of the seat is changed or adjusted. During a long time driving of the associated motor vehicle, a timer means operates to vary the air-pressure in the air-bags at regular intervals thereby to lessen the fatigue of the seat occupant.
However, in the above-mentioned conventional seat, the pressure changing operation for the air-bags during the long time driving is automatically carried out without respect to the degree of fatigue of the seat occupant. Thus, it often occurs that the pressure changing operation takes place even when the seat occupant is not so fatigued. This unnecessary operation causes the seat occupant to feel uncomfortable.
The present inventors have found that, as is depicted by the graph of FIG. 7, the fatigue of a seat occupant has a close relationship to a frequency in movement of occupant. The graph shows that as the frequency in movement of the seat occupant increases, the degree of fatigue of him or her becomes high.